This invention relates to lithographic printing. In particular, this invention relates an aqueous developer for multi-layered lithographic printing plates.
In lithographic printing, ink receptive areas, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plate precursors, often called printing plates or printing forms, typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. If after exposure to radiation the exposed regions of the coating are removed in the developing process revealing the underlying hydrophilic surface of the support, the plate is called a positive-working printing plate. Conversely, if the unexposed regions are removed by the developing process, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming more important in the printing industry. Multi-layer lithographic printing plates comprise a top layer and an underlayer on a substrate with a hydrophilic surface. After exposure to radiation, the exposed regions of the printing plate are removed during the developing process, revealing the underlying hydrophilic surface of the support.
Typically, the underlayer of these systems is soluble in the developer, but the top layer is only dispersible in the developer. The material generated by removal of the top layer in the exposed regions builds up in the developer. This material can coagulate and redeposit as sludge on the rollers of the processor and the on the walls and bottom of the developer tank. Sludge formation can severely limit developer throughput (i.e., the area of exposed printing plates developed, in m2, per liter of developer).
Thus, a need exists for a developer for multi-layered lithographic printing plates that develops multi-layer printing plates quickly, with high throughput, but with no sludge formation in the developing process.
In one embodiment, the invention is a method of developing an exposed multi-layer imageable element. The method comprises the steps of:
(A) providing an imagewise exposed imageable element comprising exposed and unexposed regions,
in which:
the imageable element comprises, in order:
a top layer;
an underlayer; and
a hydrophilic substrate;
the underlayer comprises a polymeric material that is soluble or dispersible in a developer;
the top layer comprises a polymeric material that is dispersible in the developer; and
the top layer is ink-receptive;
(B) applying the developer to the imagewise exposed imageable element, removing the exposed regions of the imageable element, and forming the image;
in which:
the developer comprises water and a dispersing agent or a mixture of dispersing agents; and
the developer has a pH of about 7 to about 11.
In another embodiment, the invention is a composition useful as a developer for multi-layered lithographic printing plates. The composition comprises:
water;
a dispersing agent or a mixture of dispersing agents;
a buffer; and
an organic solvent or a mixture of organic solvents;
in which the composition has a pH of about 7 to about 11.
Multi-layer, heat-sensitive imageable elements for the preparation of positive-working lithographic printing plates comprise at least two layers, an underlayer and a top layer, on a substrate with a hydrophilic surface. The underlayer is over the hydrophilic surface of the substrate, and the top layer over the underlayer. These systems are disclosed in, for example, in U.S. patent application Ser. No. 09/301,866 [WO 99/67097] and EP 864,420, incorporated herein by reference. Other layers, such as radiation absorbing layers may also be present in the heat-sensitive imageable element. The back side of the substrate (i.e., the side opposite the underlayer and top layer) may be coated with an antistatic agent and/or a slipping layer or matte layer to improve handling and xe2x80x9cfeelxe2x80x9d of the imageable element.
The substrate may consist only of a support, or it may additionally comprise one or more optional subbing and/or adhesion layers. The support is of sufficient thickness to sustain the wear from printing and is thin enough to wrap around a printing form. Polyethylene terephthalate or polyethylene naphthanate, typically has a thickness of from about 100 to about 310 xcexcm, preferably about 175 xcexcm. Aluminum sheet typically has a thickness of from about 100 to about 600 xcexcm. Typically, polymeric films contain a sub-coating on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface, to improve adhesion to subsequent layers, to improve planarity of paper substrates, and the like. The nature of this layer or layers depends upon the support and the composition of subsequent coated layers. Examples of subbing layer materials are adhesion promoting materials, such as alkoxysilanes, aminopropyltriethoxysilane, glycidoxypropyltriethoxysilane and epoxy functional polymers, as well as conventional subbing materials used on polyester bases in photographic films.
The underlayer comprises a polymeric material that is dispersible or, preferably, soluble in the developer, and insoluble in the solvent used to coat the top layer so that the top layer can be coated over the underlayer without dissolving the underlayer. These polymeric materials include those that contain an acid and/or phenolic functionality, and mixtures of such materials. Useful polymeric materials include carboxy functional acrylics, vinyl acetate/crotonate/vinyl neodecanoate copolymers, styrene maleic anhydride copolymers, phenolic resins, maleated wood rosin, and combinations thereof.
Solvent resistant underlayers have been developed to provide improved chemical resistance, i.e., resistance to both fountain solution and to aggressive washes. Polymeric materials useful in solvent resistant underlayers include: polyvinylacetals; copolymers that comprise N-substituted maleimides, especially N-phenylmaleimide, methacrylamides, especially methacylamide, and acrylic and/or methacrylic acid, especially methacrylic acid; aqueous alkaline developer soluble copolymers that comprise a monomer that has a urea bond in its side chain (i.e., a pendent urea group), such are disclosed in Ishizuka, U.S. Pat. No. 5,731,127; and alkaline developer soluble polymeric materials that comprise a pendent sulfonamide group, such as are disclosed in Aoshima, U.S. Pat. No. 5,141,838. Negative-working, base-soluble or dispersible photosensitive compositions, such as are disclosed in described in Baumann, U.S. Pat. No. 5,700,619, and in Photopolymers: Radiation Curable Imaging Systems, B. M. Monroe in Radiation Curing: Science and Technology, S. P. Pappas, Ed., Plenum, N.Y., 1992, pp. 399-440, may be used to produce a solvent resistant underlayer.
The top layer becomes soluble or dispersible in the developer following thermal exposure. It comprises a polymeric material that is ink-receptive and insoluble in the aqueous solution, such as acrylic and methacrylic polymers and copolymers, such as polymethyl methacrylate; polystyrene; styrene-acrylic copolymers; polyesters; polyamides; polyureas; polyurethanes; nitrocellulosic polymers; epoxy resins; or combinations thereof. Polymers that contain phenolic hydroxyl groups, such as novolac resins, resol resins, acrylic resins that contain pendent phenol groups, and polyvinyl phenol resins may be used in the top layer, either alone or in conjunction with a solubility-suppressing component.
A compound that functions as solubility-suppressing component for hydroxy-containing polymeric materials may be present in the top layer. Solubility-suppressing components have polar functional groups that are believed, to act as acceptor sites for hydrogen bonding with the hydroxyl groups. Useful polar groups include, for example, diazo groups; diazonium groups; keto groups; sulfonic acid ester groups; phosphate esters groups; triarylmethane groups; onium groups, such as sulfonium, iodonium, and phosphonium; groups in which a nitrogen atom is incorporated into a heterocyclic ring; and groups that contain a positively charged atom, especially a positively charged nitrogen atom, typically a quaternized nitrogen atom, i.e., ammonium groups. Compounds containing other polar groups, such as ether, amine, azo, nitro, ferrocenium, sulfoxide, sulfone, and disulfone may also be useful as solubility-suppressing components. These systems are disclosed, for example in West, U.S. Pat. Nos. 5,705,308 and 6,060,222; Bennett, WO97/07986 [PCT/GB96/01973]; and Nagasaka, EP 0,823,327; all of which are incorporated herein by reference.
If the printing plate is to be exposed with infrared radiation, one or both layers typically comprise an infrared absorber, known as a photothermal conversion material. An additional infrared absorbing layer that comprises a photothermal conversion material may also be present either between the substrate and the underlayer or between the underlayer and the top layer. Photothermal conversion materials include dyes and pigments, such as a dyes and pigments of the squarylium, merocyanine, indolizine, pyrilium and metal diothiolene classes, as well as carbon black. Dyes, especially dyes that are soluble in the developer, are preferred. The dye may be chosen, for example, from indoaniline dyes, oxonol dyes, porphyrin derivatives, anthraquinone dyes, merostyryl dyes, pyrylium compounds and sqarylium derivatives.
These multi-layer, thermally imageable element may be imaged with a laser or an array of lasers emitting modulated near infrared or infrared radiation in a wavelength region that is absorbed by the element. Infrared radiation, especially infrared radiation in the range of about 800 nm to about 1200 nm, is typically used for imaging a thermally imageable element. Imaging is conveniently carried out with a laser emitting at about 830 nm or at about 1056 nm. Suitable commercially available imaging devices include image setters such as a Creo Trendsetter (available from the CREO Corp., British Columbia, Canada) and a Gerber Crescent 42T (available from the Gerber Corp.).
Alternatively, the multi-layer, thermally imageable element may be imaged using a conventional apparatus containing a thermal printing head. An imaging, apparatus suitable for use in conjunction with the imageable elements includes at least one thermal head but would usually include a thermal head array, such as a TDK Model No. LV5416 used in thermal fax machines and sublimation printers.
After exposure, the exposed regions (i.e., the thermally imaged regions) of the top layer and the underlayer are removed by a developer, revealing the hydrophilic surface of the substrate. The top layer is ink-receptive, but the exposed regions of the hydrophilic surface accept water and aqueous solutions, typically a fountain solution, and repel ink.
It has been proposed that in these systems thermal exposure changes the adhesion of the top layer to the layer to which it is adhered, typically the underlayer. After imagewise thermal exposure, the developer can penetrate the exposed regions of the top layer much more rapidly than it penetrates the unexposed regions. The developing process is carried for a long enough time to remove the exposed regions of the top layer and the underlying regions of the underlayer, but not long enough to remove the unexposed regions of the top layer. The underlying regions of the underlayer are removed along with the top layer, without substantially affecting the top layer and underlayer in the unexposed region, revealing the hydrophilic surface of the substrate. Thus, the system is positive working.
Typically, only the underlayer is dissolved in the developer. The top layer is not soluble and is dispersed, rather than dissolved, in the developer. As more exposed printing plates are passed through the developer bath, the amount of material dispersed in the developer increases. Eventually, this material coagulates and deposits as sludge on the rollers of the processor and the on the walls and bottom of the developer tank. Sludge formation can severely limit developer throughput (i.e., the area of exposed printing plates developed, in m2, per liter of developer). The rollers transfer the sludge to both the image and non-image regions of the plate during development, adversely affecting the image produced by the printing plate.
It has been found that a developer comprising a dispersing agent develops multi-layer printing plates quickly, with high throughput, but with no sludge formation in the developing process. Although the specific composition of the developer will depend on the composition of the top layer and the underlayer, useful developers typically are aqueous alkaline solutions having a pH of at least 7 to about 11. Preferred aqueous alkaline developers have a pH about 8 to about 10.5, more preferably about 9 to 10, even more preferably about 10. Preferred developers are aqueous developers, i.e., those that either do not comprise an added organic solvent or to which only a small amount of organic solvent has been added.
To prevent formation of sludge during the developing process, an organic dispersing agent of mixture of agents is added to the developer. The dispersing agent forms a stable dispersion with the component or components of the top layer. The dispersing agent keeps the dispersible components of the top layer in finely dispersed form and prevents their redeposition on the exposed and developed element and on the rollers and the processor.
Dispersing agents, also known as dispersants or dispersers, are materials that are added to solid-in-liquid suspensions to separate the individual suspended particles. They are well known in, for example, the art of pigment dispersion. Organic dispersing agents may be anionic, cationic or electro-neutral. They are added to particle dispersions to prevent the particles from coagulating, to maintain the size of the particles on standing, and to prevent the particles from floating to the surface or settling to the bottom to form sludge.
Dispersing agents differ from surfactants or surface-active agents, which reduce the surface tension of liquids or reduce the interfacial tension between two liquids of a liquid and a solid.
Typical organic dispersing agents include monofunctional oleo-alkylene oxide block copolymers, such as HYDROPALAT(copyright) 1080 dispersing agent; partially neutralized acetophosphonate amine salts, such as HYDROPALAT(copyright) 3204 dispersing agent: and carboxylic acid polymers, such as Sequion MS 84 dispersing agent. Typically, about 0.5 wt % to about 10.0 wt %, preferably about 1.0 wt % to about 5.0 wt %, based on the total weight of the developer, of the dispersing agent or mixture of dispersing agents is present in the developer. Many dispersing agents are supplied as mixtures that also contain a solvent or solvents, such as water. These percentages are based on the total amount of mixture or mixtures of dispersing agents added to the developer, not on the weight of active ingredient or ingredients added to the developer. Although the amount will vary with the nature of the active ingredient or ingredients added, the amount of active ingredient or ingredients added is typically about 0.3 wt % to about 8.0 wt %, preferably about 0.5 wt % to about 4.0 wt %, based on the total weight of the developer.
Water typically comprises about 75 wt % to about 98 wt % of the developer, based on the weight of the developer; preferably about 80 wt % to 95 wt % of the developer, based on the weight of the developer; and more preferably about 85 wt % to 92 wt % of the developer, based on the weight of the developer.
The developer may also comprise a buffer system to keep the pH relatively constant in the desired pH range. Numerous buffer systems are known to those skilled in the art. Typically buffer systems include, for example: combinations of water-soluble amines, such as mono-ethanol amine, di-ethanol amine, tri-ethanol amine, or tri-i-propyl amine, with a sulfonic acid, such as benzene sulfonic acid or 4-toluene sulfonic acid; mixtures of the tetra sodium salt of ethylene diamine tetracetic acid (EDTA) and EDTA; mixtures of phosphate salts, such as mixtures of mono-alkali phosphate salts with tri-alkali phosphate salts; and mixtures of alkali borates and boric acid.
The developer may also comprise an organic solvent or a mixture of organic solvents. The organic solvent or mixture of organic solvents must be misable with water, or at least soluble in the developer to the extent it is added to the developer, so that phase separation between an aqueous phase and a phase containing the organic solvents does not occur. The following solvents and mixtures of these solvents are suitable for use in the developer: the reaction products of phenol with ethylene oxide and propylene oxide, such as ethylene glycol phenyl ether; benzyl alcohol; esters of ethylene glycol and of propylene glycol with acids having six or fewer carbon atoms, and ethers of ethylene glycol, diethylene glycol, and of propylene glycol with alkyl groups having six or fewer carbon atoms, such as 2-ethylethanol and 2-butoxyethanol. A single organic solvent or a mixture of organic solvents can be used. The organic solvent or solvent mixture is typically present in the developer at a concentration of between about 0.5 wt % to about 15 wt %, preferably about 1 wt % to about 10 wt %, more preferably about 1 wt % to about 6 wt %, and even more preferably between about 3 wt % and about 5 wt %, based on the weight of the developer.
The developer may also comprise a surfactant or wetting agent. Preferred surfactants include: alkali metal salts of alkyl naphthalene sulfonates; alkali metal salts of the sulfate monoesters of aliphatic alcohols, typically having six to nine, carbon atoms, such as sodium octyl sulfate; and alkali metal sulfonates, typically having six to nine carbon atoms. A preferred alkali metal is sodium. The developer typically comprises 1 to 10 wt %, preferably about 1 to 6 wt %, based on the weight of the developer, of the surfactant. As is well known to those skilled in the art, many surfactants are supplied as aqueous surfactant solutions. These percentages are based on the amount of surfactant (i.e. the amount of active ingredient or ingredients exclusive of water and other inactive materials in the surfactant solution) in the developer.
The developer can be used to develop printing plates, especially multi-layer thermal printing plates in which the top layer is dispersible in the developer. The developer develops multi-layer printing plates, especially two-layer printing plates, quickly, with high throughput, but with no sludge formation in the developing process.
Typically, the developer is applied to the imaged element by rubbing or wiping the top layer with an applicator containing the developer. Alternatively, the imaged element may be brushed with the developer or the developer may be applied to the element by spraying the top layer with sufficient force to remove the exposed regions. In either instance, a developed element is produced. The developing process may be conveniently carried out in a commercial processor, such as the Mercury CTP Processor (Kodak Polychrome Graphics), the Sprinter 72 Processor (Kodak Polychrome Graphics) or the Unigraph PC28E Processor (Unigraph). During the development process, the exposed regions of both the underlayer and the upper layer are removed, revealing the underlying hydrophilic substrate. The developed element, typically a lithographic printing plate or printing member, comprises (1) regions in which the underlayer and top layer have been removed revealing the underlying surface of the hydrophilic substrate, and (2) complimentary regions in which the underlayer and top layer have not been removed. The regions in which both the underlayer and top layer have not been removed are ink receptive and correspond to the regions that were not exposed during imaging.
If the underlayer comprises a negative-working base-soluble or dispersible photosensitive composition, the developed element is exposed to actinic radiation, i.e., any radiation that can induce photoinsolubilization in the underlayer. Radiation sources are well known to those skilled in the art and include, for example, lasers, fluorescent lamps, mercury, metal additive and arc lamps.
Once the imageable element has been imaged and developed, printing can then be carried out by applying a fountain solution and then a lithographic ink to the image on its surface. The fountain solution is taken up by the imaged regions and the ink is taken up by the unimaged regions. The ink is then transferred to a suitable receiving material (such as cloth, paper, metal, glass, or plastic) either directly or indirectly through the use of an offset printing blanket to provide a desired impression of the image thereon. The imaging members can be cleaned between impressions, if desired, using conventional cleaning means.